Hydride enhanced growth rates in hydride vapor phase epitaxy

ABSTRACT

Presented herein are reactors for growing or depositing semiconductor films or devices. The reactors disclosed may be used for the production of III-V materials grown by hydride vapor phase epitaxy (HVPE).

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority under 35 U.S.C. § 119 to U.S.Provisional Patent Application No. 62/617,349, filed on Jan. 15, 2018,the contents of which are hereby incorporated by reference in itsentirety.

CONTRACTUAL ORIGIN

The United States Government has rights in this invention under ContractNo. DE-AC36-08GO28308 between the United States Department of Energy andAlliance for Sustainable Energy, LLC, the Manager and Operator of theNational Renewable Energy Laboratory.

BACKGROUND

Single junction GaAs solar cells exhibit record efficiencies of 29.1%and 30.5% under one-sun and concentrated illumination, respectively.However, high material and manufacturing costs restrict III-V basedsolar cells to specialty applications such as space power and highconcentration systems, despite their many advantages over other solartechnologies. It is therefore essential to reduce the cost of III-Vepitaxial growth in order for these solar cells to reach much largermarkets. Different pathways to reduce the cost of III-V epitaxialgrowth, such as high-growth-rate metalorganic vapor phase epitaxy(MOVPE) and close-spaced vapor transport (CSVT) are currently beingstudied.

High quality single-crystal III-V materials for optoelectronicapplications such as photovoltaic devices or light-emitting diodes aretypically grown by Metalorganic Chemical Vapor Deposition (MOCVD). InMOCVD, epitaxial films of desired materials are grown by a reactionbetween methyl metalorganic precursors of the Group III constituent andhydrides of the desired Group V constituent. Thus, for example, GalliumArsenide (GaAs) may be grown on a substrate by providing pyrolyzedtrimethylgallium ((CH₃)₃Ga) and introducing arsine gas (AsH₃) into thereaction chamber. The pyrolyzed (CH₃)₃Ga leaves Ga⁺³ on the substratesurface which can react with As⁻³ in the arsine gas to growsingle-crystal GaAs films on the substrate. While capable of growinghigh-quality III-V films, however, MOCVD is problematic from acommercial perspective due to the high cost of trimethyl precursors andslow film growth which limits material growth throughput.

To address these limitations with the MOCVD growth process, attentionhas recently turned to Hydride vapor phase epitaxy (HVPE). HVPE is analternative to current standard industrial processes that has promisefor reducing the costs of III-V epitaxy. HVPE replaces the expensivegroup III metalorganic precursors used in MOVPE with lower costelemental sources, and offers the potential for higher AsH₃ utilization,and high growth rates for III-V materials without sacrificing materialquality.

HVPE is regarded for its high growth rate and its low-cost precursorinputs. In HVPE, metal chlorides (e.g. GaCl, InCl) or trichlorides (e.g.GaCl₃, InCl₃) act as the Group III transport agent and are formed fromthe in situ reaction of a halide-containing molecule (typically HCl)with the elemental Group III metal (Ga, In). Group V atoms may bedelivered by multiple mechanisms, typically Group V hydrides (AsH₃,PH₃), although an elemental (As₄) or chlorinated molecules (AsCl₃) maybe used as well.

When provided to a substrate in a reaction chamber, GaCl and AsH₃ form asurface complex of GaAsCl on the substrate which may be subsequentlyreduced by available hydrogen to form a single-crystal GaAs film andHCl(g). In the same manner, GaCl and PH₃ may form single-crystal GaP andHCl(g), while GaCl, InCl, and PH₃ may form single-crystal GaInP andHCl(g) and GaCl, InCl, and AsH₃ may form single-crystal GaInAs andHCl(g). The reduction step is thought to be the rate-limiting step ofIII-V crystal growth, with a large kinetic barrier of about 200 kJ/mol.Accordingly, HVPE deposition typically requires growth temperatures inexcess of 750° C. in order to achieve growth rates greater than 1μm/min, which is desirable for large-area/low-cost applications such asphotovoltaic devices. Temperatures in that range are generallyincompatible with the growth of important ternary alloys such as GaInP,because GaP-rich alloys are thermodynamically favored as temperaturesincrease.

It has been hypothesized that delivery of ‘uncracked’ Group V hydridessuch as AsH₃ or PH₃—that is, in the case of AsH₃, molecular AsH₃, asopposed to thermally-degraded As₂/As₄ along with a stoichiometric amountof H₂ to the growth substrate is important to increasing III-V materialgrowth rates in HVPE deposition chambers. While the precise mechanism bywhich uncracked AsH₃ and PH₃ may catalyze film growth is not wellunderstood, it is theorized that cracking of AsH₃ or PH₃ on thesubstrate surface could lead to a large number of hydrogen radicalswhich reduce Cl in, for example, the GaAsCl surface complex and therebyaccelerate the rate-limiting reduction step. Furthermore, theequilibrium constant for GaAs growth directly from AsH₃ is roughly fourorders of magnitude higher than for GaAs growth from cracked As₄.

Currently, no HVPE reaction chamber has been designed to deliveruncracked Group V hydride gases to the surface of the growth substrate.Delivering uncracked Group V hydrides to the substrate is not a trivialchallenge. Arsine, for instance, is thermally unstable at about 400° C.,which is much lower than typical HVPE deposition temperatures which maybe in excess of 700° C. Previous work by others experimented withreaction parameters to facilitate improved delivery of uncracked AsH₃ tothe substrate and demonstrated high GaAs growth rates at 700° C. withnormal reactant flow rates by reducing the reactor pressure below 8torr, leading to decreased AsH₃ residence time, and therefore a lowerpercentage of cracked AsH₃ in the reactor before reaching the substratesurface. Significant vacuum of this nature increases the complexity andexpense of the HVPE deposition process, however, while also greatlyconstraining design flexibility that may be more beneficially appliedelsewhere in the reactor.

Similarly, others have delivered PH₃ downstream (T=650° C.) of the InClinjection point (T=700° C.) rather than upstream of it, thereby avoidingpassing the PH₃ through the high temperature region of the reactor, andobserved growth rate improvements in excess of 5× typically observed,while others have observed a similar effect during GaInP growth.

Previously, Gruter et al. demonstrated GaAs growth rates up to 300 μm/hin their vacuum HVPE system. Their reactor made no attempt to deliverthe AsH₃ precursor at low temperature or at high velocity to avoidthermal cracking of AsH₃. Instead, they avoided AsH₃ cracking throughuse of extremely low reactor pressures, which enable a longmean-free-path for input precursor molecules that limits molecularcollisions and thus suppresses AsH₃ cracking. Reactor pressures below0.01 atm. were necessary to avoid cracking and achieve such high growthrates. However, a pressure that low places significant constraints onthe reactor geometry and materials that can be used in its construction,and makes the reactor more expensive to operate than an atmosphericpressure reactor.

SUMMARY

In an aspect, disclosed herein is a reactor capable of the deposition ofat least one layer of a semiconductor device by using hydride vaporphase epitaxy (HVPE), wherein the reactor is capable of producing the atleast one layer of a semiconductor device at a rate of greater than 300μm/h at a pressure of about 1 atm. In an embodiment, the reactor has agroup V hydride injector inlet, a group V hydride outlet and sourceboats. In an embodiment, the reactor has a high temperature region and alow temperature region wherein the high temperature region contains thesource boats and wherein the low temperature region contains the group Vhydride injector outlet. In another embodiment, the reactor has a hightemperature region is at a temperature of up to about 750° C., and a lowtemperature region that is at a temperature of below about 650° C. In anembodiment, the reactor has a low temperature region that is where theat least one layer of a semiconductor device is deposited. In yetanother embodiment, the reactor has at least one layer of asemiconductor device that are III-V semiconductors. In an embodiment,the reactor has a at least one layer of a semiconductor device that isselected from the group consisting of GaAs and GaInP. In anotherembodiment, the reactor has the at least one layer of a semiconductordevice that is a single-junction GaAs solar cell having an efficiency ofabout 25% or greater. In another embodiment, the reactor has the atleast one layer of a semiconductor device that has an open circuitvoltage (VOC) of greater than 1.04 V. In an embodiment, the reactor hasthe at least one layer of a semiconductor device that has a fill factorof at least 80%. In another embodiment, the reactor has the at least onelayer of a semiconductor device that contains an anti-reflectivecoating. In an embodiment, the reactor has the at least one layer of asemiconductor device that has a band gap voltage offset (WOC) of lessthan 0.4V. In yet another embodiment, the reactor has the at least onelayer of a semiconductor device that has a band gap voltage offset (WOC)of less than about 0.33V. In an embodiment, the reactor has the at leastone layer of a semiconductor device that has a EL2 trap density of lessthan about 0.4×1015 cm-3 at growth rates up to about 320 μm/h.

In an aspect, disclosed herein is a method for growing at least onelayer of a semiconductor device using a reactor having boats containinggroup III metals, a group V hydride gas, a low temperature growth regionand a high temperature region wherein the method comprises hydride vaporphase epitaxy (HVPE), and heating the group V hydride gas to atemperature of at least 750° C., and growing the at least one layer of asemiconductor device in the low temperature growth region wherein thelow temperature growth region is below about 650° C. In an embodiment,the method makes the at least one layer of a semiconductor device inwhich it is grown at a rate of greater than 300 μm/h at a pressure ofabout 1 atm. In another embodiment, the method makes the at least onelayer of a semiconductor device that has a band gap voltage offset (WOC)of less than 0.4V. In another embodiment, the method makes the at leastone layer of a semiconductor device that has a band gap voltage offset(WOC) of less than about 0.33V. In an embodiment, the method makes theat least one layer of a semiconductor device that is a single-junctionGaAs solar cell having an efficiency of about 25% or greater. In yetanother embodiment, the method makes the at least one layer of asemiconductor device that has a EL2 trap density of less than about0.4×1015 cm-3 at growth rates up to about 320 μm/h.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments are illustrated in referenced figures of thedrawings. It is intended that the embodiments and figures disclosedherein are to be considered illustrative rather than limiting.

FIG. 1 depicts a HVPE deposition chamber which is defined by chamberwalls and includes a Group V hydride injector inlet, source boats, and abottom Group V hydride injector outlet.

FIG. 2 depicts a HVPE deposition chamber which is defined by chamberwalls and includes a high temperature region and a low temperatureregion. In an embodiment, multiple Group V hydride injector outlets areincluded radially around the growth substrate in order to enhance filmgrowth uniformity.

FIG. 3 depicts a HVPE deposition chamber which is defined by chamberwalls and includes a high temperature region and a low temperatureregion. HVPE deposition chamber may include a perforated structureattached to a support structure located beneath the growth substrate.

FIG. 4 depicts a continuous HVPE deposition chamber defined by chamberwalls and has a substrate transport mechanism, two or more gas deliveryapertures, and a Group V hydride delivery channel, as well as a hightemperature region and a low temperature region. In some embodiments,the substrate transport mechanism may encircled by an inductive heatingcoil which serves as, or part of, the heating element for thelow-temperature region.

FIG. 5 depicts a device structure grown in an inverted configuration ofan embodiment of GaAs rear heterojunction solar cells disclosed herein.FIG. 5 depicts the identification of the layers in which the growth ratewas varied.

FIGS. 6(a) and 6(b) depict GaAs growth rate as a function of (a) H₂carrier flow rate through the Ga boat and (b) partial pressure ofHCl(Ga) sent to the boat. Other growth parameters are depicted in eachfigure and the total reactor flow was held constant.

FIGS. 7(a) and 7(b) depict J-V data for GaAs solar cells grown using thesame H₂Se dopant flow and a growth rate of about 180 μm/h in the contactlayer (red curve) compared with a device grown using a relatively slowgrowth rate of about 50 μm/h (black curve). FIG. 7(b) depicts a fillfactor of GaAs solar cells grown on substrates miscut 4°, 6° and 9°towards (111)B, either with the same growth rate used in the contact andbase layers, or a slower contact growth rate (blue circles).

FIG. 8 depicts EL2 trap density measured using DLTS as a function ofgrowth rate for GaAs solar cells grown by D-HVPE. Results frompreviously existing MOVPE devices are depicted for comparison.

FIGS. 9(a), 9(b), 9(c), and 9(d) depict open circuit voltage (VOC), fillfactor (FF), short circuit current (JSC) and efficiency (η),respectively, of solar cells grown at various growth rates on differentmiscut (4° B, 6° B, and 9° B) substrates. All measurements were withoutantireflection coating and under the AM1.5G illumination condition.

FIGS. 10(a) and 10(b) depict certified I-V characteristics of singlejunction GaAs cells grown at growth rate of (a) 50 μm/h on 4° B miscutsubstrate and (b) 195 μm/h on 6° B miscut substrate with a MgF₂/ZnSantireflection coating measured under the AM1.5G spectrum (certified bythe PV performance characterization team at the National RenewableEnergy Laboratory).

DETAILED DESCRIPTION

D-HVPE is a modified HVPE deposition technique in which the growthsubstrate is rapidly transferred between adjacentenvironmentally-isolated deposition chambers. D-HVPE can be used to formsuch heterointerfaces by avoiding pauses in film growth, as eachreaction chamber may be maintained at a distinct temperature. In anembodiment, disclosed herein are methods for making semiconductors usingD-HVPE at temperatures less than about 700° C. to obtain alattice-matched composition Ga_(0.5)In_(0.5)P without use of high InClor GaCl ratios to drive the reaction. The ability to grow GaAs at lowertemperatures while maintaining a high film growth rate by using D-HVPEis a cost-effective method of III-V material growth, and furthermore isa step in the manufacture of more complicated device architectures thatrequire abrupt heterointerfaces such as tandem photovoltaic devicesutilizing both GaAs and GaInP absorber layers.

Open circuit voltage (V_(OC)) is a device parameter that is indicativeof the quality of a specific material type, but, because this parameteris a function of band gap (E_(G)), using V_(OC) to compare the qualityof different materials with different E_(G) is not straightforward.Therefore, the band gap voltage offset (W_(OC)=E_(G)/q-V_(OC), where qis the elementary charge) is more commonly used to quantify materialquality in a general form that allows for comparison among differentmaterial types. W_(OC)<0.4 V is generally regarded as a threshold for“excellent” material quality. In an embodiment, semiconductor materials,such as GaAs solar cells, made using reactors and methods as disclosedherein had W_(OC) of from about 0.33V to about 0.35 V. In an embodiment,the semiconductor depicted in 10(a) has a V_(OC) of 1.08 V and a W_(OC)of 0.33 V.

In an embodiment, GaAs growth rates in excess of 300 μm/h byatmospheric-pressure D-HVPE at 650° C. are obtained using methods andreactors disclosed herein. These rates are higher than previouslyachieved by low pressure, traditional HVPE. A combination of enhancedgrowth rates and increased material utilization was achieved by usingmethods disclosed herein such as by controlling the flow of hydrogencarrier gas. The V_(OC) of single junction GaAs solar cells grown fromabout 35-309 μm/h was in the range of about 1.04-1.07 V indicating lowlevels of non-radiative recombination regardless of growth rate. Asdisclosed herein, DLTS measurements identified only EL2 traps withconcentration of less than about 3×10¹⁴ cm⁻³ and with no significantincrease with increasing growth rate, showing the quality of D-HVPEdevices grown at high rates using methods disclosed herein.

In an embodiment, reaction chambers are disclosed herein which weredesigned to optimize high quality III-V material film growth in HVPE orD-HVPE depositions by delivering to the growth substrate uncracked GroupV hydride precursors in a cost-effective manner.

In an embodiment, a reactor for deposition of multiple layers of asemiconductor device using HVPE is disclosed wherein the reactor haschamber walls, a group V hydride injector inlet, source boats, and abottom group V hydride injector outlet. In another embodiment, a reactorhas a high temperature region at a temperature of about 750° C., and alow temperature region at a temperature of about 500° C. In anotherembodiment, the reactor has one or more group V hydride injector outletslocated in the low temperature region. In another embodiment, thereactor temperature regions are heated by an optical heating source. Inan embodiment, the reactor has a deposition chamber comprising aperforated structure which is located beneath a growth substrate. In yetanother embodiment, the reactor has a perforated structure that is inthe shape of a ring, a triangle, or a square.

In an embodiment, disclosed herein are methods of performing HVPEdeposition of layers of a semiconductor device using a reactor isdisclosed wherein the method provides deposition materials and carriergas flows.

In an embodiment, an in-line, continuous reactor for deposition ofmultiple layers of a semiconductor device using HVPE is disclosed havingchamber walls, a group V hydride injector inlet, source boats, and abottom group V hydride injector outlet. In an embodiment, the reactor ofhas a high temperature region at a temperature of about 750° C., and alow temperature region at a temperature of about 500° C. In anotherembodiment, the reactor has one or more group V hydride injector outletslocated in the low temperature region. In another embodiment, thereactor has temperature regions that are heated by an optical heatingsource. In an embodiment, the reactor is configured to linearly move asubstrate via a substrate transport mechanism through one or moredeposition regions. In another embodiment, the reactor has components ofthe deposition chamber that are inverted such that the high temperatureregion is located below the substrate transport mechanism. In anembodiment, the reactor has a deposition chamber with a perforatedstructure which is located beneath a growth substrate. In yet anotherembodiment, the reactor has a perforated structure that is in the shapeof a ring, a triangle, or a square.

In an embodiment, disclosed herein are methods of performing HVPEdeposition of layers of a semiconductor device using novel reactorsusing deposition materials and carrier gas flows.

FIG. 1 is a depiction of a HVPE deposition chamber 100 which is definedby chamber walls 113 and includes a Group V hydride injector inlet 101,source boats 102, and a bottom Group V hydride injector outlet 103. TheHVPE deposition chamber may have a generally high temperature region111, which may be about 750° C., and a generally lower temperatureregion 112, which may be about 500° C. In some embodiments of theinvention, the high temperature region 111 and the low temperatureregion 112 are heated by an optical heating source which does not supplya convective or conductive heat transfer source to the depositionchamber 100. The Group V hydride precursor gas (e.g. arsine orphosphine) may be supplied to the deposition chamber 100 from anexternal source through the top Group V hydride injector inlet 101located at the top of the deposition chamber 100 and delivered, throughthe high temperature region 111, to the substrate 104 from the bottomGroup V hydride injector outlet 103 as shown in flow pathway 109. Insome embodiments, the Group V hydride source may be refrigerated orotherwise configured to deliver low temperature Group V hydride gas tothe Group V hydride injector inlet 101. In an embodiment, the Group Vhydride injector outlets are located above the source boats. In anotherembodiment, the Group V hydride injector outlets are located above theGroup III metal. GaCl or InCl may be delivered to the substrate bypassing HCl(g) from an external source (not shown) through HCl inlet 105to either or both source boats 103, which contain the Group III metal(e.g. Ga, In) 106. The HCl(g) reacts with the Group III metal to form,for example, GaCl(g), which exits the source boat 102 through the sourceboat aperture 107 and recirculates into the deposition chamber 100 inthe high temperature region 111 behind HCl inlet 105 as shown by flowpathway 108. In some embodiments of the invention, HCl(g) flow pathway108 may be independently heated such that HCl(g) or reacted GaCl(g) orInCl(g) is delivered to the deposition chamber 100 at a temperatureabove the ambient in the high-temperature region 111. The HCl(g) flowpathway 108 and Group V hydride flow pathway 109 may converge onsubstrate 104 in the lower temperature region 112 to deposit the desiredIII-V material (e.g. GaAs) 110.

In some embodiments, deposition chamber 100 may be inverted, such thatsubstrate 104 and low-temperature region 112 are located above thehigh-temperature region 111 and Group V hydride injector outlet 103. Inthis embodiment, Group V hydride flow pathway 109 and HCl(g) pathway 108flow upward from below the substrate 104 to deposit the desired III-Vmaterial 110 on the bottom of substrate 104.

Multiple HVPE deposition chambers 100 may be environmentally isolatedand placed in series, which may enable the more efficient deposition oflayers comprising distinct material compositions and varioustemperatures by moving the substrate sequentially through individualchambers. For example, a thick photovoltaic absorber layer may be grownin a deposition chamber 100 configured to maximize the III-V material110 growth rate, while a subsequently grown thinner layer (e.g. a tunneljunction) may be grown in another discrete, serially-connected,deposition chamber 100 configured to more precisely control the III-Vmaterial's 110 growth rate. Serially connected deposition chambers 100may be controllably environmentally connected, such that precursor gaspresent in one deposition chamber 100 may controllably flow to one ormore subsequent serially connected deposition chambers 100. For example,uncracked Group V hydride precursor gas present in a first depositionchamber 100 may be delivered to one or more subsequent depositionchambers 100 in order to more efficiently utilize the stock of initialprecursor material.

In an embodiment, reactors disclosed herein are capable of growth in upto about one atmosphere. In other embodiments, reactors disclosed hereinare capable of growth in up to about ten atmospheres or more. In anotherembodiment, higher quality films are grown at lower temperatures thanthose previously disclosed.

FIG. 2 is a depiction of a HVPE deposition chamber 200, which is definedby chamber walls 211 and includes a high-temperature region 208 and alow temperature region 209. In some embodiments of the invention, thehigh temperature region 208 and the low temperature region 209 areheated by an optical heating source which does not supply a convectiveor conductive heat transfer source to the deposition chamber 200. HVPEdeposition chamber 200 includes one or more Group V hydride injectoroutlets 201, which are located in the low temperature region 209 ofdeposition chamber 200. By moving the Group V hydride injector outlets201 into the lower temperature region, as well as closer to the growthsubstrate 203, it is expected that delivery of uncracked Group V hydrideprecursor gas to the growth substrate 203 will be enhanced. Moreover, itmay be desirable that multiple Group V hydride injector outlets 201 beincluded radially around the growth substrate 203 in order to enhancefilm 210 growth uniformity.

In all other aspects, deposition chamber 200 functions similarly todeposition chamber 100, in that GaCl or InCl may be delivered by passingHCl(g) from an external source through HCl inlet 204 to either or bothsource boats 202, which contain a Group III metal (e.g. Ga, In) 205. TheHCl(g) reacts with the Group III metal to form, for example, GaCl(g),which exits the source boat 202 through the source boat aperture 206 andrecirculates into the deposition chamber 200 in the high temperatureregion 208 behind HCl inlet 204 as shown by flow pathway 207. In someembodiments of the invention, HCl(g) flow pathway 207 may beindependently heated such that HCl(g) or reacted GaCl(g) or InCl(g) isdelivered to the deposition chamber 200 at a temperature above theambient in the high temperature region 208. The HCl(g) flow pathway 207can then converge on substrate 203 with the Group V hydride suppliedthrough Group V hydride injector outlet 201 in the lower temperatureregion 209 to deposit the desired III-V material (e.g. GaAs) 210. As indeposition chamber 100, the Group V hydride is supplied from an externalsource which may be refrigerated or otherwise configured to deliverlow-temperature Group V hydride to the Group V hydride injector outlet201.

In some embodiments, deposition chamber 200 may be inverted, such thatsubstrate 203 and low temperature region 209 are located above the hightemperature region 208 and Group V hydride injector outlet 201. In thisembodiment, HCl(g) pathway 207 flows upward from below the substrate 203to deposit the desired III-V material 210 on the bottom of substrate203.

Multiple deposition chambers 200 may be environmentally isolated andplaced in series which may enable the more efficient deposition oflayers having distinct material compositions and various optimal growthtemperatures by moving the substrate sequentially through individualchambers. For example, a thick photovoltaic absorber layer may be grownin a deposition chamber 200 configured to maximize the III-V material210 growth rate, while a subsequently grown thinner layer (e.g. a tunneljunction) may be grown in another discrete, serially connected,deposition chamber 200 configured to more precisely control the III-Vmaterial 210 growth rate. Further, serially connected depositionchambers 200 may be controllably environmentally connected, such thatprecursor gas present in one deposition chamber 200 may controllablyflow to one or more subsequent serially connected deposition chambers200. For example, uncracked Group V hydride precursor gas present in afirst deposition chamber 200 may be delivered to one or more subsequentdeposition chambers 200 in order to more efficiently utilize the stockof initial precursor material.

FIG. 3 is a depiction of a proposed HVPE deposition chamber 300, whichis defined by chamber walls 313 and includes a high temperature region310 and a low temperature region 311. In some embodiments, the hightemperature region 310 and the low temperature region 311 are heated viaan optical heating source which does not supply a convective orconductive heat transfer source to the deposition chamber 300. HVPEdeposition chamber 300 includes a perforated structure 301 attached to asupport structure 302, which is located beneath the growth substrate305. Perforated structure 301 may include multiple perforations 303through which Group V hydride can be supplied from an external source tothe deposition chamber 300 in the low temperature region 311. In someembodiments, the Group V hydride source may be refrigerated or otherwiseconfigured to deliver low temperature Group V hydride to the multipleperforations 303. Perforated structure 301 can be of any desired shapeincluding, for example, a ring, a triangle, a square, or any other shapewhich may be desirable for uniformly depositing the desired III-Vmaterial 312. Similarly, perforations 303 may be of any size or shaperequired in order to supply Group V hydride precursor gas to thedeposition chamber 300 in the necessary quantity or flow rate. Moreover,the configuration of perforations 303 may be in any shape or patternwhich is designed to increase the spatial uniformity or otherwisecontribute to the improved growth of the desired III-V material 312.Perforated structure 301 may be located at any depth below growthsubstrate 305 which facilitates improved growth of III-V material 312 atthe chosen growth temperature, flow rates, etc. In some embodiments,placing the perforated structure 301 at a location above the growthsubstrate 305 will be desirable.

In an embodiment, deposition chamber 300 functions similarly todeposition chambers 100 and 200, in that GaCl or InCl may be deliveredby passing HCl(g) from an external source through HCl inlet 306 toeither or both source boats 304, which contain the Group III metal (e.g.Ga, In) 307. The HCl(g) reacts with the Group III metal to form, forexample, GaCl(g), which exits the source boat 304 through the sourceboat aperture 308 and recirculates into the deposition chamber 300 inthe high temperature region 310 behind HCl inlet 306 as shown by flowpathway 309. In some embodiments of the invention, HCl(g) flow pathway309 may be independently heated such that HCl(g) or reacted GaCl(g) orInCl(g) is delivered to the deposition chamber 300 at a temperatureabove the ambient in the high temperature region 310. The HCl(g) flowpathway 309 can then converge on substrate 305 with the Group V hydridesupplied through the perforations 303 on perforated structure 301 in thelower temperature region 311 to deposit the desired III-V material (e.g.GaAs) 312.

In some embodiments, deposition chamber 300 may be inverted, such thatsubstrate 305 and low temperature region 311 are located above the hightemperature region 310. In an embodiment, HCl(g) pathway 309 flowsupward from below the substrate 305 to deposit the desired III-Vmaterial 312 on the bottom of substrate 305. The perforated structure301 may be located above or below the substrate 305.

Multiple deposition chambers 300 may be environmentally isolated andplaced in series which may enable the more efficient deposition oflayers comprising distinct material compositions, at varioustemperatures, by moving the substrate sequentially through individualchambers. For example, a thick photovoltaic absorber layer may be grownin a deposition chamber 300 configured to maximize the III-V material312 growth rate, while a subsequently grown thinner layer (e.g. a tunneljunction) may be grown in another discrete, serially-connected,deposition chamber 300 configured to more precisely control the III-Vmaterial's 312 growth rate. Further, serially connected depositionchambers 300 may be controllably environmentally connected, if desired,such that precursor gas present in one deposition chamber 300 maycontrollably flow to one or more subsequent serially-connecteddeposition chambers 300. For example, uncracked Group V hydrideprecursor gas present in a first deposition chamber 300 may be deliveredto one or more subsequent deposition chambers 300 in order to moreefficiently utilize the stock of initial precursor material.

An in-line, continuous deposition chamber 400 is shown in FIG. 4.Continuous deposition chamber 400 is defined by chamber walls 414 andcomprises a substrate transport mechanism 401, two or moreGaCl(g)/InCl(g) delivery apertures 402, and a Group V hydride deliverychannel 403, as well as a high temperature region 409 and a lowtemperature region 404. In some embodiments of the invention, the hightemperature region 409 and the low temperature region 404 are heated viaan optical heating source which does not supply a convective orconductive heat transfer source to the deposition chamber 400. Infurther embodiments of the invention, the substrate transport mechanism401 may encircled by an inductive heating coil which serves as, or partof, the heating element for the low temperature region 404.

The substrate transport mechanism 401, located in a low temperatureregion 404, may be configured to linearly move a substrate 405 throughone or more deposition regions 406, 407, and 408. In some embodiments ofthe invention, substrate transport mechanism 401 is a conveyor belt. Infurther embodiments of the invention, the substrate transport mechanism401 may be located above the Group V hydride delivery channel 403 andmay be permeable to gas flow for facile film growth. EachGaCl(g)/InCl(g) delivery aperture 402 is located in a high temperatureregion 409 and may be configured to deliver GaCl(g) and/or InCl(g) tothe continuous deposition chamber 400. In some embodiments,GaCl(g)/InCl(g) delivery aperture 402 may operate substantially the sameas deposition chambers 100, 200, and 300, provided the GaCl(g) and/orInCl(g) is here delivered to the deposition chamber 400 at large (asopposed to directly to the substrate) and does not include a Group Vhydride component. In an embodiment, one or more sequential depositionregions 406, 407, and 408 may be characterized by each having a GaCl(g)or InCl(g) flow rate, Group V hydride or PH₃ flow rate, temperature,substrate residence time, substrate transport mechanism 401 path length,or any other unique growth parameter configured to preferentiallydeposit a III-V material at the desired quality, composition, spatialuniformity, or other desirable film characteristic.

Group V hydride delivery channel 403, located in low-temperature region404, may be a simple conduit configured to permit fluid flow of Group Vhydride precursor gas, from an external source. In some embodiments, theGroup V hydride source may be refrigerated or otherwise configured todeliver low temperature Group V hydride precursor gas to the Group Vhydride delivery channel 403. Group V hydride delivery channel 403 mayinclude one or more perforations 410 which are of a number, size, andshape configured to deliver a desired amount of Group V hydrideprecursor gas to the substrate 405. In some embodiments, Group V hydridedelivery channel 403 is configured such that the rate of Group V hydridesupplied to the substrate 405 is controllable in each deposition region406, 407, and 408.

GaCl(g) or InCl(g) supplied by GaCl(g)/InCl(g) delivery aperture 402from high temperature region 409 may interact with Group V hydrideprecursor gas supplied by Group V hydride delivery channel 403 in thelow temperature region 404 at the substrate 405 in a first desireddeposition region 406 to deposit a first III-V material 411. After thefirst III-V material 411 is deposited, the substrate 405 can be movedalong the substrate transport mechanism 401, to a second depositionregion 407, in which a second III-V material 412 may be deposited.GaCl(g) or InCl(g) may be supplied by either the same or a uniqueGaCl(g)/InCl(g) delivery aperture 402 as may be desired for thecomposition or quality of the second III-V material 412. After thesecond III-V material 412 is deposited, the substrate 405 can be furthermoved along the substrate transport mechanism 401 to a third depositionregion 408, in which a third III-V material 413 may be deposited aspreviously set forth in deposition regions 406 and 407. In someembodiments, there may be n sequential deposition regions correspondingn layers of desired III-V materials in the completed device stack.

In further embodiments of deposition chamber 400, the components ofdeposition chamber 400 may be inverted, such that the GaCl(g)/InCl(g)delivery aperture 402 and the high temperature region 409 are locatedbelow the substrate transport mechanism 401 and Group V hydride deliverychannel 403 in the low-temperature region 404. In this embodiment,substrate 405 and Group V hydride delivery channel 403 may be located onthe bottom of substrate transport mechanism 401 to facilitate precursorgas flow to the substrate 405 surface. Substrate 405 may be affixed tothe substrate transport mechanism 401 by any suitable mechanical,chemical, or other means as may desirable.

In an embodiment GaAs growth rates in excess of 300 μm/h were obtainedusing D-HVPE with GaAs solar cells grown at these rates showinginsignificant degradation in V_(OC), which was used as a proxy foroverall material quality, compared to devices grown using lower rates.

In an embodiment, growth experiments were performed in a dual-chamberD-HVPE system. The sources used in the D-HVPE system were AsH₃ and PH₃for the group V sources, and GaCl and InCl, which are formed in situ byflowing anhydrous HCl over Ga and In metal, as the group III sources. Inan embodiment, the dopants used were Zn (p-type) and Se (n-type)supplied as diethylzinc and H₂Se, respectively. The source zone wherethe metal chlorides are formed was held at 800° C., while the depositionzone (growth temperature, T_(G)) was held at 650° C. for materialsgrown.

Growth rate studies were conducted by growing lattice matchedGaAs/GaInP/GaAs structures on (100) GaAs substrates miscut 4° towards(111)B. Growth rates were determined by selectively etching a portion ofthe top GaAs layer to the GaInP etch stop and using the GaAs thicknessmeasured using stylus profilometry and the known growth time. Changes ingrowth rate were studied as functions of gas flows in the reactor,including GaCl flow and the flow of H₂ carrier gas injected intodifferent parts of the system.

In an embodiment, single junction GaAs solar cells were grown in aninverted configuration with lattice matched GaInP window and backsurface field (BSF) layers in a rear heterojunction design. In anembodiment, FIG. 5 depicts a structure with targeted thickness anddoping of each layer made using the devices, techniques and methodsdisclosed herein. As depicted in FIG. 5, GaInP layers were grown at 2.3μm/h for the etch stop and window and 6 μm/h for the BSF layers, andwere the same for all experiments. In an embodiment, the growth ratesexemplified herein refer to GaAs contact and base layers, as depicted inFIG. 5, for example. Solar cells were grown on (100) GaAs substratesoffcut either 4°, 6° or 9° toward (111)B at growth rates from 35-309μm/h. Device processing proceeded wherein Au was electroplated on theBSF, serving as a back contact as well as a back reflector to increasethe optical path length of the device. This Au back contact was bondedto a Si handle using epoxy and the GaAs substrate and GaAs:Se bufferlayer were removed by a selective chemical etch that stops at the GaInPetch stop layer. The front Au grid formation and 0.25 cm² square mesaisolation for solar cells and 0.7 mm² rectangular arrays fortransmission line measurements were completed using standardphotolithography processes. On selected samples, a MgF₂ (100 nm)/ZnS (52nm) antireflection coating (ARC) was deposited in a thermal evaporator.

External quantum efficiency and reflectance were measured using a custominstrument which was used to calibrate a XT10 solar simulator to the airmass 1.5G spectrum, under which current density-voltage (J-V)characteristics of the devices were measured. Deep-level transientspectroscopy (DLTS) measurements were performed on select samples toquantitate the effect of growth rate on the trap type and density. TheDLTS system used herein uses Fourier transforms to characterize fullcapacitance transients with a reverse bias voltage of −5.0 V, a trapfilling pulse of 0.70 V and a pulse width of 1.0 msec for thesemeasurements. The DLTS measurements were performed on 0.7 mm² deviceswith the same structure as the solar cells.

Growth Rate Determination

There are several factors that affect the growth rate in themass-transport-limited HVPE growth parameter space. In an embodiment,the first is the efficiency of the GaCl conversion reaction from HCl andGa. In an embodiment, the efficiency is governed by the temperature inthe source region, 800° C., for example, but also by the residence timeof the HCl in the Ga boat if the kinetics of the HCl to GaCl conversionreaction are not sufficiently fast. The HCl residence time in the Gasource boat is defined predominantly by the flow of H₂ carrier gas thatpushes the HCl through the boat, Q_(H) ₂ ^(Ga).

FIG. 6(a) depicts an embodiment in which reducing the H₂ flow ratethrough the Ga boat is useful as a method for determining specificgrowth rates over a wide range while changing only one parameter. Thereduced carrier flow permits the generation of more GaCl for a given HClflow rate and thus minimizes the amount of free HCl in the reactor. Inan embodiment, increased use of reactants was achieved because free HCl,which would otherwise suppress the growth rate through the reversereaction (etching of GaAs), is reduced. As depicted in FIG. 6(a), allother growth parameters, i.e. the substrate temperature, the HCl flowrate through the Ga boat, the AsH₃ flow, and the total system H₂ carrierflow of about 10250 sccm, were held constant. Decreasing Q_(H) ₂ ^(Ga)from 2000 sccm to 75 sccm increased the growth rate from about 50 μm/hto about 100 μm/h. Without being limited by theory, the increase in thegrowth rate is attributed to an increase in the conversion of Ga to GaClas the H₂ flow rate, and gas velocity through the Ga boat is reduced.

In an embodiment, the second factor affecting growth rate is the masstransport of reactants to the growth surface. FIG. 6(b) depicts theeffect of increasing the GaCl partial pressure by varying the HCl flowrate through the Ga boat, using the Q_(H) ₂ ^(Ga)=75 sccm conditions asdiscussed above. The green data in FIG. 6(b), which use a H₂ carrier gasflow through the AsH₃ inlet, Q_(H) ₂ ^(AsH) ³ =3125 sccm, show aninitial increase in growth rate when the GaCl partial pressure increasesfrom 0.8 to 1.7×10⁻³ atm. However, for higher GaCl partial pressures thegrowth rate may stagnate and then slightly decrease. This likelyindicates that the growth rate is limited by a process whereby GaClcomplexes compete with As species for adsorption sites at higher GaClpartial pressure.

In an embodiment, a third way to increase growth rate is to increase theAsH₃ carrier flow rate. FIG. 6(b) depicts that increasing Q_(H) ₂ ^(AsH)³ to 5000 sccm increases the growth rate for a given partial pressure ofGaCl. The same saturation in growth rate is observed as for lower Q_(H)₂ ^(AsH) ³ , but the effect is delayed to higher P_(HCl(Ga)). Withoutbeing limited by theory, this could be due to the additional AsH₃providing hydrogen radicals that reduce the Cl-containing complexes intoHCl, or to the larger source of available As for a given supply of Ga,although the exact mechanism is currently unclear. In an embodiment, thegrowth rate increase was related only to an increase in Q_(H) ₂ ^(AsH) ³with constant GaCl and AsH₃ partial pressures resulting in greatersource utilization efficiency at these high-growth-rate conditions.

In an embodiment, combining the three effects as discussed above, amaximum growth rate of about 320 μm/h at a GaCl partial pressure of2.4×10⁻³ atm was obtained which is an improvement over existing GaAsgrowth rates of 300 μm/h obtained in different HVPE system havingpressure of about 0.10 atm. Thus, in an embodiment, the greater than 300μm/h growth rates obtained by using the methods disclosed herein atatmospheric pressure is beneficial at least because high vacuumconditions impose stricter design requirements on reactor materials andphysical shape, and are typically more expensive to operate thanatmospheric pressure systems disclosed herein.

Solar Cell Performance as a Function of Growth Rate

The effect of high growth rate on solar cell design and material qualityby using high-growth-rate MOVPE GaAs solar cells is demonstrated, forexample, by measuring Si and Zn dopant incorporation efficiency in theGaAs and was shown to increase as the growth rate increased requiringaltering of dopant flows. FIG. 7(a) compares the J-V curves of twoD-HVPE-grown GaAs solar cells with GaAs:Se contact layers grown atdifferent growth rates, but with the same H₂Se flows. The fill factor ofthe device with the high-growth-rate contact layer suffers fromincreased series resistance compared to the device with the slowercontact layer. Transmission line measurements indicated that theincreased series resistance occurs at the metal/semiconductor interface,and not elsewhere in the device. Without being limited by theory, thiseffect is likely due to a simple reduction in volumetric dopant densitybecause the same amount of dopant disperses in more matrix materialdeposited per unit time, and, in an embodiment, can be an impediment tomaking high-efficiency devices.

In an embodiment, single junction GaAs solar cell structures were grownwith the same H₂Se dopant flow but different growth rates to investigatethis effect. FIG. 7(b) depicts the fill factor measured from the J-Vcurves for samples grown on (100) substrates miscut 4° toward (111)B(black circles) as a function of growth rate. The same growth rate wasused for the contact and the base layer in this sample set. In anembodiment, this is the simplest arrangement in the D-HVPE reactordisclosed herein because constant reactant flows can be used in the GaAsdeposition chamber for the entire device run. The fill factor decreaseswith contact growth rate, as depicted in FIG. 7(a).

In an embodiment, three methods were used to recover lost fill factorthat focused on the region of contact growth rates at around 180 μm/h.The first was to increase the H₂Se dopant flow from 6 sccm to 12 sccm(gold circles in FIG. 7(b)), which increased the fill factor to greaterthan 80%, up from about 70%. The second method was to use a hybridgrowth rate structure, where base layers were grown at rates of about180 μm/h, but the contact layer growth rate was slowed to about 60 μm/h.This approach requires a change in the reactant flows in the GaAsdeposition chamber after deposition of the contact layer, before thebase is grown. This approach was similarly effective (blue circles inFIG. 7(b)). The last approach was to use higher substrate miscuts (6° Band 9° B) than 4° B substrates. Without being limited by theory, theadditional group V terminated surface steps/kinks present with thesehigher miscuts enhance the adsorption and incorporation of Se atoms. Toverify this effect, a set of GaAs epilayers were grown on substrateswith varying miscut towards (111)B with a H₂Se flow of 6 sccm and agrowth rate of 100 μm/h. Carrier concentrations, derived from roomtemperature Hall effect measurements, increased from 5.0×10¹⁸ to1.0×10¹⁹ to 1.3×10¹⁹ cm⁻³ when increasing the miscut from 4° to 6° to9°, respectively. The fill factors of the solar cells grown with 6° Band 9° B miscut substrates with 6 sccm H₂Se flow and 180 μm/h contactlayers (green and red circles in FIG. 7(b)) are also about 10%(absolute) higher than the devices with 4° B miscuts.

In an embodiment, DLTS measurements were performed on GaAs devices grownby D-HVPE at rates from 50-309 μm/h on 4° B miscut substrates and ondevices grown at 180 μm/h on 6° B and 9° B miscut substrates todetermine the effect of growth rate on EL2 concentrations in HVPE. Theactivation energy of the traps observed in the D-HVPE-grown samples wasabout 0.82 eV, indicating an EL2 defect, and no other traps wereidentified. FIG. 8 depicts the measured EL2 trap density for D-HVPE GaAssolar cells as a function of both growth rate and miscut angle. There isno clear trend with substrate miscut and, unlike the MOVPE data, thetrap density does not vary significantly with growth rate. As depictedin FIG. 8, the substrates grown using methods and reactors disclosedherein exhibited EL2 trap densities of less than about 0.4×10¹⁵ cm⁻³ atgrowth rates up to about 320 μm/h.

To verify the quality of high growth rate HVPE material, a series ofGaAs solar cells were grown using D-HVPE with widely varying growthrates (from about 35-309 μm/h). The time required to grow a 2 μm-thickbase layer at 309 μm/h was approximately 23 s, which is about 20×shorter than what it takes at a standard MOVPE growth rate. FIG. 9displays the open-circuit voltage (VOC), fill factor (FF), short circuitcurrent density (JSC), and efficiency (Eff) extracted from J-Vmeasurements for the full series of solar cells as a function of growthrate, using rates from about 35-309 μm/h on 4° B miscut substrates, andfrom about 84-200 μm/h on 6° B and 9° B miscut substrates in which noneof the devices had an ARC applied.

FIG. 9(a) depicts the VOCs for all of these devices fall in the rangefrom 1.04-1.07 V. Without being bound by theory, VOC is an indicator ofthe quality of the active layers in a solar cell because it reflectsnegative contributions from non-radiative recombination and thesedevices are only about 50-80 mV lower than record GaAs devices. Thedevices were not optimized at each growth rate, i.e. the same growthrecipe was used for each of the samples shown in FIG. 9 up to about 200μm/h. The one change made in devices grown at rates greater than about200 μm/h was to increase the H₂Se flow in order to counteract thepreviously observed decrease in Se incorporation. Without being bound bytheory, although this approach led to very comparable results in termsof all cell performance metrics, i.e., open-circuit voltage, fillfactor, short-circuit current density and efficiency, continuedoptimization of the very-high-growth rate cells may lead to essentiallyidentical performance for the entire range of growth rates.

FIG. 10 depicts two NREL-certified I-V measurements comparing (a) a GaAssolar cell grown on a 4° B miscut substrate at 50 μm/h with (b) asimilar solar cell grown on a 6° B miscut substrate at 195 μm/h, withboth devices achieving efficiencies of about 25%. As discussed above,the change in the miscut angle only helped to enhance the dopantincorporation and improves the contact resistance, but does nototherwise affect device performance as can also be seen in FIG. 9. FIG.10 depicts, however, the insensitivity of the D-HVPE process to thelarge growth rate range. Thus, in an embodiment, in D-HVPE the growthrate has no appreciable influence on the performance of solar cells.

The foregoing disclosure has been set forth merely to illustrate theinvention and is not intended to be limiting. Since modifications of thedisclosed embodiments incorporating the spirit and substance of theinvention may occur to persons skilled in the art, the invention shouldbe construed to include everything within the scope of the appendedclaims and equivalents thereof.

What is claimed is:
 1. A method for growing at least one layer of asemiconductor device using a reactor comprising boats containing groupIII metals, a group V hydride gas, a low temperature growth region and ahigh temperature region wherein the method comprises hydride vapor phaseepitaxy (HVPE), and heating the group V hydride gas to a temperature ofat least 750° C., and growing the at least one layer of a semiconductordevice in the low temperature growth region wherein the low temperaturegrowth region is below 650° C.; and wherein the at least one layer of asemiconductor device is a single-junction GaAs solar cell having anefficiency of 25% or greater.
 2. The method of claim 1 wherein the atleast one layer of a semiconductor device is grown at a rate of greaterthan 300 μm/h at a pressure of 1 atm.
 3. The method of claim 1 whereinthe at least one layer of a semiconductor device has a band gap voltageoffset (W_(OC)) of less than 0.4V.
 4. The method of claim 1 wherein theat least one layer of a semiconductor device has a band gap voltageoffset (W_(OC)) of less than 0.33V.
 5. A method for growing at least onelayer of a semiconductor device using a reactor comprising boatscontaining group III metals, a group V hydride gas, a low temperaturegrowth region and a high temperature region wherein the method compriseshydride vapor phase epitaxy (HVPE), and heating the group V hydride gasto a temperature of at least 750° C., and growing the at least one layerof a semiconductor device in the low temperature growth region whereinthe low temperature growth region is below 650° C.; and wherein the atleast one layer of a semiconductor device has a EL2 trap density of lessthan 0.4×10¹⁵ cm⁻³ at growth rates up to 320 μm/h.
 6. The method ofclaim 5 wherein the at least one layer of a semiconductor device isgrown at a rate of greater than 300 μm/h at a pressure of 1 atm.
 7. Themethod of claim 5 wherein the at least one layer of a semiconductordevice has a band gap voltage offset (W_(OC)) of less than 0.4V.
 8. Themethod of claim 5 wherein the at least one layer of a semiconductordevice has a band gap voltage offset (W_(OC)) of less than 0.33V.
 9. Themethod of claim 5 wherein the at least one layer of a semiconductordevice is a single-junction GaAs solar cell having an efficiency of 25%or greater.